AND FRIEDRICH WIDDEL1* Max-Planck-Institut für Marine Mikrobiologie, D-28359 Bremen,1

Enrichment and pure cultures of nitrate-reducing bacteria were shown to grow anaerobically with ferrous iron as the only electron donor or as the additional electron donor in the presence of acetate. The newly observed bacterial process may significantly contribute to ferric iron formation in the suboxic zone of aquatic sediments. Iron is the most prevalent redox-active metal in the bio-sphere. In anoxic soils and sediments, ferric iron is reduced to the ferrous state mainly through the activities of various iron-reducing bacteria (15, 18). Widespread and well-studied reoxi-dation processes of iron(II) are its chemical reactions with molecular oxygen under neutral conditions (21) and its biolog-ical utilization as an electron donor by aerobic acidophilic (1